First let me say that I personally have never slept with anyone on a first date.
I know plenty of women who have, though, and then they complain that they can't find a guy who wants to seriously date them. I think the answer is pretty simple. The guys are seeing them as easy and they don't see them as girlfriend-material... rather just someone to have a good time with for a while.
Now I'm sure this isn't always the case. For example, like someone already pointed out, if it's someone you've known for a while then maybe it's not so bad. But when a woman goes out and meets a guy for the first time and winds up in bed with him that night, what reason does he have to respect her? Just my two cents
